In accordance with recent technical developments, various types of personal computers, such as desktops, towers and notebooks, are being produced and sold. Notebook computers, designed for outdoor use while taking into consideration the mobility and portability, are compact and light. A typical example of such a notebook computer is the "IBM ThinkPad 770" series sold by IBM Japan, Co., Ltd ("IBM ThinkPad 770" is a trademark of IBM Corporation).
Almost all the notebook PCS can be operated by incorporated batteries because this type of PC can even be employed in a mobile environment where AC power is not available. An incorporated battery is provided as a "battery pack" in which are stored a plurality of rechargeable battery cells, such as Li-Ion, NiCd or NiMH battery cells.
While the capacity of an AC power source is generally considered to be unlimited, the capacity of the battery pack is always limited and its operating period is short (usually, at most, two to three hours), and an extended period of time is required to recharge it (generally, the time required for charging is the equivalent of the operating period for the battery). A user can carry a spare battery to extend the effective battery operating period; however, since a spare battery is inconvenient, heavy and bulky, portability is degraded. Therefore, in an office environment where AC power is available, an external AC adaptor is connected to a notebook computer to drive it using the AC power. The AC adaptor is a device for transforming an AC voltage into a DC voltage, and includes a rectifier and a transformer for transforming the level of a DC voltage, which is well known. A cable extending from one end of an AC adaptor is inserted into an AC outlet (normally built into a wall in a room), and another cable extending from the other end of the AC adaptor is inserted into a DC inlet in the case of a notebook PC. Power output by the AC adaptor is used to drive the apparatus as is described above, and extra power or power supplied in the power-off state is used to recharge an incorporated battery.
Recent notebook PCS are capable of replacing desktops, i.e., can serve as "desktop replacements." This is because, as the semiconductor fabrication techniques have been developed, the processing capabilities of notebook computers have been improved until they are equivalent to those of desktop PCS, and also because the size of an LCD panel screen and the number of drive units have been increased and the working environment for notebook PCS is as excellent as is that for the desktop PCS. In addition, since a notebook PC has a smaller volume and a small footprint, installation space in an office environment can be saved.
When a notebook PC is used only in an office, an AC power source is employed, as is described above. In most cases, a notebook PC that is used in this manner is constantly connected to an AC outlet, even when it is not in use (e.g., at night and on holidays). However, when the apparatus is in the power OFF state or the AC adaptor is removed from the apparatus, the AC adaptor connected to the AC outlet continues to be conductive, so that a problem arises concerning power consumption during this period. While an AC adaptor includes a transformer for transforming a DC voltage, control, using an analog switch such as an FET switch, is performed on the primary coil side of the transformer in order to stabilize the voltage output. Regardless of whether a PC is powered off or the AC adaptor is removed from the PC, so long as the AC adaptor is inserted into the outlet and is receiving AC power, the switching control is constantly performed. That is, power consumed by the AC adaptor is mainly that required by the switching control. In addition, since most of the power loss is converted into thermal energy, a countermeasure is also required that will disperse heat from the AC adaptor.
The following is a calculation of the power consumed by an AC adaptor not in use. As an example, a current resonant AC adaptor, which is used for a typical notebook PC, the "IBM ThinkPad" series, consumes 2 to 4 W of power because of switching loss, even when the PC is in the power off state, and a "flyback" AC adaptor consumes 0.5 to 1 W of power. Generally, since current resonant AC adaptors have superior voltage conversion efficiency, they tend to be used instead of the flyback AC adaptors, even though when not in use, the power loss for the current resonant type is greater. Further, since even inside a PC a DC voltage (e.g., 16 V) is constantly applied by an AC adaptor, approximately 0.5 W of power is consumed in the power off state. Although the power loss attributable to a single PC is relatively minute, being approximately 3 W, at an office wherein a large number of PCS are installed, merely by keeping AC adopters attached to AC power sources, a considerable charge for electricity can be accrued.
Assuming that the period of time during which PCS are not used at night on weekdays is 12 hours, and that the period of time during which PCS are not used on weekends is 60 hours. The total time in a year that a PC is not used is 1,280 hours/year (=12 hours/day.times.200 days/year+60 hours/week.times.4 weeks/month.times.12 months/year). Therefore, if there were one million notebook PCS connected to AC adopters, the accompanying annual power loss would be 15,840,000 KWh/year (=1,000K units.times.1,280 hours/year.times.3 W), and assuming, e.g., the charge for electricity is 239,184K yen (=0.0151K .Yen.Y/KWh/year.times.15,840,000 KWh/year).
Since an AC power source is unlimited, from the standpoint of supplying power for driving PCS, practically no problems exist. However, when consideration is given to the sociobiological aspect, i.e., from the viewpoint of the effective use of resources and of environmental protection globally, such a power loss as that attributable to the AC adaptors that are not in use state can not be ignored.
In order to eliminate the loss of power at an AC adaptor not in use state, it is preferable that the AC adaptor be detached from an outlet and a notebook PC each time after it has been used. However, the connection and the disconnection of cables are burdensome tasks, and they contribute to the deterioration of usability. In addition, the frequent disconnection of an AC adaptor accelerates the wear and tear experienced by an outlet and the plug of an AC adaptor. And also, since a notebook PC recharges its incorporate battery using power supplied in the power-off state, a user must keep an AC adaptor attached.
Another method may be employed whereby in a state where the output from the AC adaptor is unnecessary (i.e., a period during which the main box of a PC is powered off and is not being recharged) a signal to this effect is output by a notebook PC, and upon the receipt of this notice, the AC adaptor cuts off power to the primary coil side of the transformer. In IBM Technical Disclosure Bulletin No. JA8-97-0299, for example,there is disclosed an invention that embodies a method for adding a new signal line along which a notebook PC transmits the power supply state signals to an AC adaptor to halt the operation of the AC adaptor. However, the addition of a signal line is accompanied by a loss of the connection compatibility at a DC inlet that connects the PC and the AC adaptor. In other words, the notebook PC according to IBM TDB JA8-97-0299 accepts only the AC adaptor that is described in that disclosure. Also, this AC adaptor can be employed only for that notebook PC according to IBM TDB JA8-97-0299.
A technique for avoiding power loss due to an AC adaptor when an apparatus is not in use is disclosed in Japanese Unexamined Patent Publication Nos. Hei 6-292362, Hei 4-165957, Hei 7-153582 and Hei 8-179858. However, since in Japanese Unexamined Patent Publication No. Hei 6-292363 a battery in the apparatus is used to turn on the primary side switch of an AC adaptor, the operation of the AC adaptor can not be halted if there is no battery incorporated in the apparatus or if there is no power remaining in an incorporated battery.
The invention disclosed in Japanese Unexamined Patent Publication No. Hei 4-165957 detects the presence of a load current in an AC adaptor to control the on/off state of the AC adaptor. Therefore, when an apparatus that has no incorporated battery is powered on, the AC adaptor can not detect a change (or an event) in the state wherein the supply of power should be initiated. That is, there occurs a contradiction that affects the AC adaptor and prevents it from being turned on again.
In Japanese Unexamined Patent Publication No. Hei 7-153582 there is disclosed a power saving technique for an AC adaptor used for luminaries. In this publication, the AC adaptor is integrally formed with a luminary, and the power saving technique for an AC adaptor that is connected to the main box of an apparatus is not taught.
In Japanese Unexamined Patent Publication No. Hei 8-179858, a power switch is provided for an AC adaptor. When this power switch is manually operated, the AC adaptor can be turned on and off, without removing the AC adaptor from an AC outlet. However, the AC adaptor can not be turned off automatically. For example, it is impossible for the AC adaptor to be turned off automatically in response to completion of the recharging process for the apparatus in the power-OFF state.